Marriage began when God created Eve to be a wife for Adam.
Then the man said, “This at last is bone of my bones and flesh of my flesh; she shall be called Woman,because she was taken out of Man.”
Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.
The first marriage was a union between the first man and the first woman that God created. Ever since then, marriage has been considered a union between a man and a woman. This isn’t just a Christian belief but a practice by all religions and all cultures throughout history. There have been different beliefs about some aspects of marriage. Some cultures consider marriage a permanent union while other allow a person to divorce his spouse and marry someone else. Some consider it a union between one man and one woman while others allow a man to have more than one wife. But until recently it has universally been a state that can exist only between a man and a woman.
Now there is a movement to establish “marriage equality” between gay and straight people by legalizing marriage between two people of the same sex. Is the restriction of marriage to a man and a woman just an arbitrary rule or is there some fundamental reason why two members of the same sex can’t marry each other?